1. Field of the Invention
The present invention relates to a sheet conveying device capable of executing switchback conveyance of a sheet.
2. Description of the Related Art
Some copying machines, facsimile apparatuses, multi function printers (to be referred to as an MFP hereinafter) or document scanners include an automatic document reading apparatus (to be referred to as an ADF hereinafter) that feeds stacked document sheets one by one and reads image information. Some of the ADFs include a sheet conveying device capable of automatically inverting a document in the device and reading its both surfaces. As one of the arrangements of the ADFs, a document whose one surface has been read is inverted via a switchback conveyance path to read the other surface.
From the viewpoint of cost reduction, an arrangement has recently been demanded which implements the switchback operation by driving the ADF using a single driving source without using any expensive electrical components such as an electromagnetic clutch and a solenoid plunger. In addition, since the product needs to be compact to save the space, the ADF also reduces its size. As a result, the first half portion of a document that has been led in and read and the second half portion of the document yet to be read pass by each other in the same path. It is therefore necessary to separate the rollers for switchback.
Japanese Patent Laid-Open No. 2007-230657 discloses an ADF that performs switchback conveyance to invert a document, thereby reading both surfaces of the document. The separation means for separating a pair of discharge rollers is formed from a cam and a lever that swings along the cam face. The cam face is provided so as to drive the separation means at such a timing that allows the document to reach a pair of conveyance rollers at the time of switchback. There is disclosed a method of mechanically controlling, using the cam face, the timing the document reaches the pair of conveyance rollers from the start of switchback until the pair of discharge rollers separate and the motor rotates in the reverse direction and the timing the pair of discharge rollers separate in the above-described arrangement.
In the technique of Japanese Patent Laid-Open No. 2007-230657, however, if the document slips during switchback or is misaligned at the start of switchback due to an error of the document edge detection sensor or the like, the document may be unable to reach a pair of intermediate rollers (for example, the pair of conveyance rollers) at the assumed timing. In this case, a pair of forward/reverse rotation rollers (for example, the pair of discharge rollers) may separate and lose the conveyance capability before the pair of intermediate rollers nip, resulting in document jam. To prevent this, the delay time set for the cam face and the time until the pair of intermediate rollers nip need to be exactly managed. This leads to severe requirements of dimensional accuracy of cam parts and conveyance accuracy of rollers and thus makes it difficult to increase the productivity.
Setting a longer delay time for the cam face allows to solve the above-described problem. However, since the pair of forward/reverse rotation rollers do not separate for some time after passing through the nip of the pair of intermediate rollers, the document needs to be conveyed by reverse rotation for some time up to the position where the forward/reverse rotation of the motor can be switched. If the step in which the rotation direction of the motor needs to be switched is to be arranged next to the pair of intermediate rollers, it is necessary to separate the step from the pair of intermediate rollers. This may make it difficult to simultaneously implement the arrangement and downsizing of the device.